Rare earth diphthalocyanines are organometallic complexes many of which are known to have electrochromic properties, i.e., their color changes in an applied electric field as a result of either oxidation or reduction of the molecule. Perhaps the best known of these substances is lutetium diphthalocyanine, whose chemical formula is commonly abbreviated to LuH(Pc).sub.2 where Pc=[(C.sub.32 H.sub.16 N.sub.8).sup.2- ]. A proposed structure of this substance is shown in FIG. 1 in which it can be seen that the lutetium atom is sandwiched between two large diphthalocyanine rings of the kind known as a macro cycle and the metal diphthalocyanine molecule itself is known as a macro cycle complex. The lutetium diphthalocyanine complex can take any charge state of +2 to -1. These charge states correspond to a wide range of colors: red (+2), green (+1), blue (0) and violet (-1). A distinct yellow-tan color is visible between the red and the green states. The singly positively charged (green) radical cation is the normal stable state and has an anion, the nature of which is not known, associated with it for charge neutrality.
Early studies of these properties are described in an article by P. N. Moskalev and I. S. Kirin entitled "Effect of the Electrode Potential on the Absorption Spectrum of a Rare Earth Diphthalocyanine Layer," (Optika I Spectroskopy Vol. 29, p. 220, 1970). This paper gives experimental results on the marked color changes of lutetium diphthalocyanine with applied potential in an electrolytic cell. In the experimental arrangement, the diphthalocyanine was coated over an electrode consisting of a transparent film of semiconductive tin oxide on a glass plate and placed in a 0.1 molar aqueous K Cl electrolyte. The potential of the electrode was varied relaive to a saturated calomel electrode and the resulting color changes observed. Other diphthalocyanines were also studied.
Published German patent application No. 2756551 (Rockwell International Corp.) describes various electrochromic display cells in which the electrochromism is provided by a metal diphthalocyanine layer. In an example, a lutetium diphthalocyanine layer is formed over tin oxide display electrodes deposited on a glass substrate. The film may be incorporated in a resin binder. The glass substrate is one wall of a sealed chamber in which counter and reference electrodes are also provided. The chamber is filled with plaster of paris for a reflective display and with a transparent gel for a transmissive display. The filler medium is saturated with aqueous potassium chloride as electrolyte, though other salts are also mentioned as possible alternatives.
It has been found that when such a lutetium diphthalocyanine display is cycled between different color states, the color intensity decreases gradually until it is no longer observable. In some cases, the film of lutetium diphthalocyanine even peels off the underlying electrode. The maximum number of cycles obtainable before such degradation takes place is less than 20-50,000.
Besides metal diphthalocyanines, several other solid film electrochromic materials are known. A great deal of work has been carried out on certain inorganic metal oxides, particularly tungsten and molybdenum oxides. These, too, are generally formed as layers over underlying display electrodes and color changes are brought about by the application of an electric potential between the display and a counter electrode through an electrolyte. A display employing these two substances is described in U.K. Pat. No. 1,356,120 which discusses, inter alia, the properties of the electrolyte which may be liquid or semi-solid (gels) and comprise dissolved acids or salts for conductivity. Sulfuric acid or salts of lithium are mentioned as suitable conductivity agents which are compatible with the tungsten oxide material and the counter electrodes. A preferred electrolyte is a gel of sulfuric acid in polyvinyl alcohol though other solvents such as water, glycerine and ethylene glycol are also mentioned.
It was later discovered that these types of electrolytes were unsuitable for tungsten oxide displays because of dissolution of the tungsten oxide and because of damage to the electrodes caused by hydrogen evolution which could take place at potentials not very different from those associated with the color change. Various subsequent patents (U.K. Pat. No. 1,535,594, U.K. published patent application 3,014,326, U.S. Pat. Nos. 4,110,015 and 4,139,275) suggest alternative organic solvents which are proton (hydrogen ion) free, do not have side reactions and do not lengthen switching time of the electrochromic films (i.e., the time taken to change color).